Coating compositions are used in a variety of applications to coat a variety of substrates, often for protection of the substrate or to improve adhesion of subsequent coating layers. Typical coatings include electrodeposition coatings, primers, sealers, basecoats, clearcoats, and one-coat topcoats. Coating compositions include film-forming materials containing one or more resins, which may be polymeric, oligomeric, and/or monomeric materials, that are applied to a substrate by various methods, including electrodeposition (or electrocoating), spray coating, dip coating, roll coating, knife coating, and curtain coating. As used herein, a “resin” refers to one or more polymeric, oligomeric, and/or monomeric materials; a polymer includes repeating monomer units; an oligomer includes a few repeating monomer units, typically ten or fewer. Various types of film-forming materials are known, including epoxy, acrylic, polyurethane, polycarbonate, polysiloxane, aminoplast, and polyester resins.
Coating compositions can include a pigment dispersing or grind resin and a principal resin that generally constitutes the major polymeric part of the coating film. A grind resin usually includes a film-forming material, with which a pigment paste is made by wetting out pigment, filler, and catalyst, such as a metal catalyst, where the grind resin is blended or mixed with the other materials by milling in, e.g., a sandmill, ball mill, attritor, or other equipment. The pigment paste is combined with the principal resin and, typically, a curing agent. The grind resin and the principal resin can include the same, different, or mixtures of various film-forming materials.
The relatively soft film of an applied coating composition can be hardened by curing or crosslinking the film through incorporation of a crosslinker or curing agent in the coating composition. The crosslinker can be chemically reactive toward the polymers, oligomers, and/or monomeric compounds of the resin in the coating composition, thereby covalently joining the film-forming units together into a crosslinked film. Typical crosslinkers are activated (e.g., unblocked) using heat during a curing step and/or by exposure to actinic radiation. Catalysts, such as metal catalysts, can be used to facilitate thermal activation of the crosslinker and the reaction of the crosslinker with the resin. For example, inclusion of a catalyst such as a metal catalyst can reduce the requisite cure temperature and/or provide for a more complete cure.
Coating compositions can be powder, organic solvent based, or aqueous based. However, it is often desirable to use aqueous based coatings in order to reduce organic emissions. Such aqueous coating compositions include emulsions and dispersions of cationic, anionic, or nonionic resins, which may be formed via the dispersive properties of the resins themselves or with aid of external surfactants.
Epoxy-based coatings include polymers, oligomers, and/or monomers prepared by reacting materials with epoxide groups with materials having functional groups such as carboxyl, hydroxyl, and amine groups. Epoxies can be cured or crosslinked to form hardened coatings by using various crosslinkers depending on the functional groups present. For example, hydroxy-functional resin can be cured using isocyanate compounds. Such coating compositions are known in the art; e.g., U.S. Pat. Nos. 6,852,824; 5,817,733; and 4,761,337.
The electrodeposition process can be anodic or cathodic; typically the article to be coated serves as the cathode. Electrodeposition processes are advantageous both economically and environmentally due to the high transfer efficiency of coating resin to the substrate and the low levels of organic solvent, if any, that are employed. Another advantage of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even and continuous coating layer formed over all portions of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat baths can comprise an aqueous dispersion or emulsion of a film-forming material, such as an epoxy resin, having ionic stabilization. A dispersion is typically a two-phase system of one or more finely divided solids, liquids, or combinations thereof in a continuous liquid medium such as water or a mixture of water and organic cosolvent. An emulsion is a dispersion of liquid droplets in a liquid medium, preferably water or a mixture of water and various cosolvents. Accordingly, an emulsion is a type of dispersion.
For automotive or industrial applications, the electrocoat compositions are formulated to be curable compositions by including a crosslinker. During electrodeposition, a coating composition containing an ionically-charged resin is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin and then applying an electrical potential between the substrate and a pole of opposite charge, for example, a stainless steel electrode. The charged coating particles are plated or deposited onto the conductive substrate. The coated substrate is then heated to cure the coating.
It is desirable to increase the performance of coating compositions. Particularly, for many applications, improvement in the adhesive strength of the cured coating film would be beneficial. Furthermore, reducing the cure temperature for crosslinking the coating film would simplify the coating process by reducing the energy and expense required. Moreover, lower cure temperatures would be advantageous for applying coatings to thermally-sensitive substrate materials. Finally, any simplification in the synthesis and preparation of coating compositions that reduces time and expense would provide further advantages.
A need, therefore, exists for coating compositions that have better substrate adhesion, reduced cure temperatures, and that are simpler to produce.